Robert Mugabe (l) and Morgan Tsvangirai (r) have been in a fractious coalition since 2009
Zimbabwe's President Robert Mugabe has acted "unconstitutionally" by declaring an election date, Prime Minister Morgan Tsvangirai has said.
Mr Mugabe set the polls for 31 July - the deadline given by the Constitutional Court.
Mr Tsvangirai has been pushing for key reforms as a condition for elections.
After four years of coalition government, the two men are set to resume their bitter rivalry in the presidential poll.
Mr Tsvangirai, 61, pulled out of the second round of the previous election, in 2008, accusing the security forces and pro-Mugabe militias of attacking his supporters around the country.
Mr Mugabe, 89, denied any links to the violence.
He named the election date using a presidential decree to bypass parliament, reports the Reuters news agency.
"Given the deadline imposed by the Constitutional Court, it is inexpedient to await the passage through Parliament of an act dealing with the situation," it quotes Mr Mugabe as saying in an official government notice.
Mr Tsvangirai told journalists in the capital, Harare, that Zimbabwe could not hold elections before 25 August, Reuters reports.
"President Mugabe is acting unlawfully and unconstitutionally," Mr Tsvangirai said.
"As prime minister I cannot and will not accept this."
Mr Mugabe had previously said he would abide by the court ruling and the coalition had "outlived its usefulness".
However, Mr Tsvangirai said the court should be asked to review its decision, as more time was needed to prepare for elections. 'Challenge of interpretation'
Parliament is still considering a draft electoral law, under which all parties would be given access to the state broadcaster, which is generally seen as a mouthpiece for Mr Mugabe's Zanu-PF party.
Mr Tsvangirai's Movement for Democratic Change (MDC) has also been pushing for reforms to the security agencies, which it says are allied to Zanu-PF.
Trevor Maisiri, southern Africa analyst at the International Crisis Group think tank, said the 2009 power-sharing deal requires Mr Mugabe to consult Mr Tsvangirai on an election date, AFP news agency reports.
"The interpretation of the word consultation is a bit of a challenge," he is quoted as saying.
The MDC argues that it means decisions have to be taken jointly, but Zanu-PF says that Mr Mugabe can decide "with their suggestions", Mr Maisiri said.
Five years ago, Mr Tsvangirai won the most votes in the first round of the presidential election but, according to official results, not enough to win outright.
He pulled out of the second round, saying his supporters were being targeted in a campaign of violence.
After Mr Mugabe went ahead with the election, winning with 85% of votes cast, regional mediators intervened to organise the power-sharing agreement.
Mr Mugabe has been in power since independence in 1980.